Light sources with Light Emitting Diodes are known. A plurality of Light Emitting Diodes (LEDs) are commonly available in the form of LED arrays. Such LED arrays have LEDs providing light of the same wavelength. However, if it is required that a single LED array have LEDs of various wavelengths, a custom LED array must be assembled This assembly of custom LED arrays begins with wafers containing a plurality of LEDs of the same wavelength. The wafers are cut, separating units of LEDs from one another. These units of LEDs are called dice. A single die may contain one or more LEDs (of the same wavelength) and an associated circuitry. Dice from different wafers may contain LEDs of different wavelength. Dice containing various wavelength LEDs are assembled into an LED array in order to provide an LED array with custom spectral characteristics.
More specifically, the dice are placed onto a conductive substrate using precision positioning and bonding equipment. This equipment allows extremely small dice (for example, 500 microns in width) to be assembled very precisely on a substrate and bonded into place. The precision of die location is on the order of plus or minus tens of microns. A bonding wire is usually attached to each die to complete an electrical circuit. This method of assembly is expensive because of the cost of the precision equipment required, and because multiple operations (such as attaching bonding wires) are performed serially. Furthermore, the sequence of operations performed by the equipment is specific to the type of an LED array being assembled. Therefore, when a configuration of an LED array is changed, new cost is incurred because the equipment needs to be reset and reprogrammed. Thus, this method of assembly is difficult to practice, except by very specialized, high volume manufacturing facilities.
An alternative method of assembling custom LED arrays requires the use of discrete LED packages of various wavelengths. These LED packages come in long strips (also referred to as tapes). The individual LED packages are removed from the strips, and are positioned and bonded to a circuit board in a specific sequence in order to achieve the spectral mix and spectral uniformity desired. The positioning and bonding process is sometimes a manual operation, and is therefore subject to error in the placement and sequencing of the LED packages. More often, this process requires the use of expensive precision placing and assembly equipment. This method of positioning and bonding is expensive because of the cost of the precision equipment. Regardless whether a manual assembly or an assembly utilizing the expensive precision equipment is used, the preferred method of bonding the LED packages to a circuit board is soldering. During the soldering process the individual LED packages tend to move slightly, either due to mechanical pressure of the soldering instrument or due to surface tension of the molten solder. Thus, after the solder hardens, the individual LED packages are often slightly misaligned with respect to one another. These misalignments result in LED arrays that provide non-uniform spectral illumination.
Individual LED packages and LED arrays can be used to illuminate an integrating cavity. The U.S. Pat. No. 5,548,120 also discloses that the individual LEDs can be modulated to ensure adequate brightness uniformity at an exit port of the integrating cavity. The primary disadvantage of mounting LED arrays within an integrating cavity is that each integrating cavity configuration must be individually determined to efficiently mount a particular LED array internal to the integrating cavity. This must be done in such a way that the LED array assembly does not absorb light because light absorption reduces the efficiency and brightness uniformity of the integrating cavity. The problems are compounded if LED packages of various wavelengths are required to achieve a specific color balance. Different color LED packages are often configured differently, thus requiring a different assembly for LED packages of different color. This renders the optimum mounting of these devices within the integrating cavity very difficult and expensive.